Elastomeric fluoropolymers (i.e. fluoroelastomers) exhibit excellent resistance to the effects of heat, weather, oil, solvents and chemicals. Such materials are commercially available and are most commonly either dipolymers of vinylidene fluoride (VF.sub.2) with hexafluoropropylene (HFP) or terpolymers of VF.sub.2, HFP, and tetrafluoroethylene (TFE). While these di- and terpolymers have many desirable properties, including low compression set and excellent processability, their low temperature flexibility is not adequate for all applications.
It is known that incorporation of perfluorinated ether monomer units into vinylidene fluoride elastomers improves low temperature properties. For example, Carlson, in U.S. Pat. No. 5,214,106 discloses that when perfluoro(methyl vinyl)ether (PMVE) is substituted for HFP, the resultant VF.sub.2 /PMVE/TFE copolymers have glass transition temperature (T.sub.g) values which are 10.degree.-20.degree. C. lower than those of the corresponding VF.sub.2 /HFP/TFE copolymers. T.sub.g is often used as an indicator of low temperature flexibility because polymers having low glass transition temperatures maintain elastomeric properties at low temperatures.
Kruger, in U.S. Pat. No. 5,696,216, discloses PMVE-containing fluoroelastomers that are similar to those disclosed by Carlson. Those disclosed by Kruger contain copolymerized units of VF.sub.2 ; at least one fluorinated prepends and or fluorinated methyl vinyl ether; TFE; at least one perfluoro(polyoxyalkyl vinyl)ether, and a crosslinking site.
The compositions of Carlson and Kruger are most effectively crosslinked through use of peroxide cure systems. However, when compression molding equipment is used with peroxide curable VF.sub.2 /PMVE copolymers the compositions generally exhibit a tendency to stick to and foul the mold.
Tetrapolymers of VF.sub.2, HFP, TFE and perfluoro(alkyl vinyl)ethers (PAVE) other than PMVE are also known to exhibit improved low temperature properties compared to terpolymers of VF.sub.2, HFP and TFE. For example, Arcella, et al. in U.S. Pat. No. 5,260,393 disclose a tetrapolymer comprising copolymerized units of 48-65 wt. % VF.sub.2, 21-36 wt. % HFP, 3-9 wt. % PAVE, and 0-17 wt. % TFE. The compositions can be cured using a bisphenol curing system and do not exhibit the mold fouling problems associated with peroxide cures of VF.sub.2 /PMVE copolymers. Similarly, British Patent 1,296,084 discloses fluoroelastomeric tetrapolymers containing copolymerized units of 48-65 wt. % VF.sub.2, 8-23 wt. % HFP, 4-15 wt. % TFE, and 17-30 wt. % PAVE. Such compositions have good low temperature properties and are curable with bisphenols or amines. Although these tetrapolymers exhibit good low temperature properties, many applications require improved low temperature and processability performance.
Merely raising the PAVE content while lowering the HFP content is not a solution to the problem of improving low temperature performance of VF.sub.2 /HFP/PAVE/TFE terpolymers. This is because polymers wherein the level of HFP is below about 8-10 mole percent do not contain sufficient copolymerized monomer sequences consisting of HFP units flanked by VF.sub.2 units to permit efficient crosslinking with bisphenols. As is well known in the art, efficient curing of VF.sub.2 /HFP-containing fluoroelastomers with a bisphenol/accelerator system is possible only when a ----CH.sub.2 -group in the polymer backbone is flanked by two perfluorinated carbons (e.g. CF.sub.2 CF(CF.sub.3)CH.sub.2 CF.sub.2 CF.sub.2), rendering the hydrogens acidic enough to be abstracted by base. The dehydrofluorinated polymers are easily crosslinked by bisphenols. Furthermore, as discussed by W. W. Schmiegel, in Angewandte Makromolekulare Chemie, 76/77, 39 (1979), completely eliminating HFP to form VF.sub.2 /TFE/PMVE terpolymers results in formation of monomer sequences consisting of TFE/VF.sub.2 /TFE; TFE/VF.sub.2 /PMVE; PMVE/VF.sub.2 /PMVE; and PMVE/VF.sub.2 /TFE. Although such sites readily undergo elimination of HF or HOCF.sub.3 in the presence of base, the double bonds thus formed are not easily crosslinked by bisphenols or any other traditional crosslinking agents.
There thus exists an unfulfilled need in the art for a method of providing copolymers of VF.sub.2, TFE, and PAVE that maintain optimum low temperature properties, but which exhibit low mold sticking characteristics, improved processability and are easily curable.